Basically, you figure out what matters and what doesn't by screwing it up. Try, fail, learn, try again. If you are conscious and pay attention, you can pick up on the patterns.
E.g., I still remember a rug I did not buy in Turkey, because I was in college and $250 was a lot of money at the time. I still wish I had bought it. 10 years later when I fell in love with a piece of art in Greece on my honeymoon, I bought it even though I thought it was too much and DH had to tote it around the rest of the trip, and that puppy has had pride of place in every home we have owned. But I also remember how happy I am with a new car, and how that excitement fades after about the first 6 mos. or a year when the newness fades away. Ergo, special, unique things that I fall in love with are probably worth it; things that are appealing because they are bright/shiny/new are not.
Services: I have services clean my house and do my lawn care, both because of allergies/asthma and because we are busy and I hate that shit. OTOH, I am a good cook, and I had gotten too reliant on takeout and hot dogs and other quick/easy crap. And I started to feel bad -- like when you do something that you know is wrong but you do it anyway. So I finally listened to that inner voice and am re-upping my game on that front, re-imposing some standards, cutting back on the spend, taking more time to plan and do. And I can already feel those muscles coming back. It feels good, which tells me I'm on the right track.